Method for determining consistent water relative permeability values from dynamic displacement data

ABSTRACT

Corrected water relative permeability values for a core sample are determined from dynamic displacement measurements by the steps of plotting a ratio of water rate output (q w ) to calculated water relative permeability values (k rw ) vs. average water saturation (S w ), determining if a straight line segment exists in areas of the plot corresponding to high average oil saturations, and if it does, extrapolating the straight line segment to the end of the plotted data, otherwise plotting a tangent to the plotted ratio from a beginning point of the plot, producing a line in parallel with the extrapolated straight line segment or the tangent line but shifted so as to pass through the irreducible water saturation point S iw , and correcting the calculated water relative permeability values k rw  by multiplying the calculated values k rw  by a ratio of a value on the q w  /k rw  plot to a value on the constructed straight line. The applied corrections eliminate distortions in the calculated water relative permeability values k rw  due to an end effect at the core sample which causes distortions in the dynamic displacement measurements.

CROSS-REFERENCE TO RELATED APPLICATION

The present invention is related to subject matter disclosed and claimedin U.S. application Ser. No. 544,175, filed on even date herewith andassigned to the same assignee as the present invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for determining consistentwater relative permeability values (k_(rw)) from dynamic displacementmeasurements conducted on a subsurface core sample.

2. Discussion of the Prior Art

Water and oil relative permeability values are used in a number ofsignificant ways in many water and oil reservoir engineeringcalculations. However, it is different to measure water and oil relativepermeabilities in the laboratory on a core sample. Two techniques aretypically used by the oil industry to obtain water and oil relativepermeability values. The first is the so-called steady state method,which is described in the article entitled "Relative PermeabilityMeasurements on Small Core Samples" by Morris, R. H. et al, TheProcedures Monthly, pp. 19-25, August, 1947. The other method is thedynamic displacement method, which is described in the followingarticles: "A Simplified Method for Computing Oil Recovery by Gas orWater Drive" by Wedge, H. J., Transactions, AIME, Vol. 195, pp. 91-98,1952; "Calculation of Relative Permeability From DisplacementExperiments" by Johnson, E. F. et al, Transactions, AIME, Vol. 216, pp.370-372, 1959; "Graphical Techniques for Determining RelativePermeability From Displacement Experiments" by Jones, S. C. et al,Transactions, AIME, Vol. 265, PP. 807-817, 1978. This latter articlediscloses a graphical technique for determining relative oil (k_(ro))and water (k_(rw)) permeability values from dynamic displacementmeasurements, which technique will be hereinafter referred to as theJones and Roszelle technique.

A significant problem with using a steady state technique to determinewater relative permeability values is that it is time consuming, as thesteady state measurements required take considerable time forstabilization. Thus, a few days may be required for every data point ofa plot of water relative permeability vs. core saturation (water (S_(w))or oil (S_(o))) which is calculated in the technique and thus weeks arerequired to obtain a complete water relative permeability curve.

In the dynamic displacement method of calculating water relativepermeability values, a small core sample is flooded with water tosaturation and then flooded with oil to its irreducible watersaturation. This cycle is repeated while the pressure drop across thecore, and the oil and water production fractions, as a function of totaloil and water injected (injection rate x time), are recorded. This data,together with oil and water viscosity, the absolute permeability of thecore, and the core pore volume, are used to calculate oil relativepermeability values (k_(ro)), as well as water relative permeabilityvalues (k_(rw)), as a function of saturation (oil or water) at theeffluent end of the core. A greater appreciation of this conventionaltechnique can be had by review of those above-referenced articles whichdiscuss the dynamic displacement method.

The theory upon which interpretation of the dynamic displacement datarests and upon which water relative permeability determinations are madeassumes that the capillary pressure effects on the core saturationdistribution are negligible. However, there is an observable pressuredrop discontinuity near the effluent end of the core sample causd bycapillary forces, which is known as the "end-effect". This resistance toeffluent flow distorts the pressure drop data which are taken during adynamic displacement test, causing consequent distortions in bothrelative oil and water permeability values determined from that data.

The so-called "end-effect" was perhaps first recognized by Richardson,et al and reported in the article entitled "Laboratory Determination ofRelative Permeability", Richardson, J. G. et al, Transactions, AIME,Vol. 195, pp. 187-196; 1952. In this article, Richardson, et al reportexperimental results of a two-phase flow on a 30 centimeter long coresampler cut into 8 sections. The sections were arranged perpendicular tothe axis of the core and were machined and clamped together in a flowapparatus. Two-phase flow experiments were run at various flow rates andthe saturaton in each section was measured for each experiment. Theresults show that near the outflow or effluent end of a core sample, azone exists where the wetting phase saturation increases rapidly andachieves a maximum value at the effluent face. Beyond this zone, towardsthe inflow end, the saturation is uniform. The width of the zonedecreases with an increase in flow rate.

It appears from the results reported by Richardson, et al that a veryhigh flow rate is required in the dynamic displacement tests toeffectively remove the end zone and its effects on the dynamicdisplacement data used for determining relative water permeabilityvalues. However, physical limitations on the experimental equipment usedin the dynamic displacement measurements typically do not permit thehigh rates of flow which would be required to eliminate measurementperturbations caused by the end-effect. Moreover, it is difficult todetermine a critical minimum flow rate which is required to eliminatethese perturbations.

SUMMARY OF THE INVENTION

One object of the present invention is to provide a method foraccurately determining water relative permeability values from dynamicdisplacement measurement data. In particular, the invention is directedto a method in which water relative permeability values are firstcalculated, using conventional techniques, such as, for example, theJones and Roszelle graphical technique discussed above, fron dynamicdisplacement measurement data, and then corrected to eliminate coreend-effect perturbations. Thus, with the method of the inventionconsistent water relative permeability values can be obtained fromdynamic displacement core sample measurement data, even when lower fluidflow rates are used.

This object of the invention is achieved by first calculating waterrelative permeability values k_(rw) from dynamic displacementmeasurement data, using, for example, the Jones and Roszelle techniquediscussed above. Thereafter a ratio q_(w) /k_(rw) is plotted againstaverage water (S_(w)) or oil (S_(o)) saturation (S_(w) =1-S.sub. o,where q_(w) is the water flow rate at the output end, that is, q_(w)=f_(w) q_(t), where f_(w) is the instantaneous water fraction in theeffluent leaving the core (a value between 0 and 1) and q_(t) is therate of oil injection into the core during an oil flood. In plottingq_(w) /k_(rw), an initial starting point is selected which is acalculation of k_(rw) at the residual water saturation of the core,using Darcy's Law. For this point, f_(w) approaches zero and the valueq_(w) /k_(rw) approaches zero. The irreducible water saturation (S_(iw))of the core is also determined.

The plotted value q_(w) /k_(w) vs. average water (Shd w) or oil (S_(o))saturation is then examined. It normally exhibits a straight linesegment in the range of high oil saturation. A straightline segmentparallel to the exhibited straight line segment, but shifted so that itpasses through the irreducible water saturation point (S_(iw)) is drawnand extrapolated to the limit of the experimental data obtained from thedynamic displacement experiments. For very low displacement rates, thestraight line segment may not exist, in which case a first tangent curveis drawn to the curve of q_(w) /k_(rw) vs. average saturation (waterS_(w) or oil S_(o)), beginning at the lowest water saturation point. Astraight line segment is then drawn from the irreducible watersaturation value S_(iw) and in parallel with a straight tangent line tothe plotted ratio to the extent of the experimental data. Followingthis, corrected water relative permeability values are calculated inaccordance with the expression k_(rwc) =k_(rw) (q_(w) /k_(rw))/(q_(w)/k_(rw))SL, where (q.sub. w /k_(rw)) is the plotted q_(w) /k_(rw) valueusing the water relative permeability values k_(rw) obtained from, e.g.,the Jones and Roszelle technique, and (q_(w) /k_(rw))SL is the plottedvalue read on the straight line for the same average saturation value.

The value k_(rwc) is the corrected water relative permeability value,which is used in the place of the water relative permeability valuesk_(rw) to obtain a curve of water relative permeability vs. saturationat the effluent end of the core sample.

The method of the invention, as well as the advantages thereof, will bemore readily understood from the following detailed description which isprovided in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 are plots of: (1) a ratio of q_(w) /k_(rw) vs. average watersaturation (S_(w)) for a first core sample and flow rate q_(t), producedusing water relative permeability values k_(rw) calculated in accordancewith conventional techniques; and (2) a correction line produced inaccordance with the method of the invention;

FIG. 2 are plots of: (1) (q_(w) /k_(rw)) vs. average water saturation(S_(w)) for the first core sample at a second flow rate q_(t), producedusing water relative permeability values k_(rw) calculated in accordancewith conventional techniques; and (2) a correction line produced inaccordance with the method of the invention;

FIG. 3 are plots of: (1) (q_(w) /k_(rw)) vs. average water saturation(S_(w)) for the first core sample at a third flow rate q_(t), producedusing water relative permeability values k_(rw) calculated in accordancewith conventional techniques; and (2) a correction line produced inaccordance with a method of the invention;

FIG. 4 are plots of water relative permeability values k_(rw),calculated using conventional techniques, vs. water saturation (S_(w))for different displacement rates (q_(t)) for the first core sample;

FIG. 5 are plots similar to those of FIG. 4, but showing corrected waterrelative permeability values k_(rwc) obtained in accordance with themethod of the invention;

FIG. 6 are plots similar to those of FIG. 4, but for a second coresample;

FIG. 7 are plots of corrected k_(rwc) values vs. water saturation S_(w)similar to those of FIG. 5, but for the second core sample;

FIG. 8 are plots, similar to those of FIGS. 4 and 6, but for a thirdcore sample;

FIG. 9 are plots of corrected k_(rwc) values vs. water saturation S_(w)similar to those of FIGS. 5 and 7, but for the third core sample;

FIG. 10 are plots similar to those of FIGS. 4, 6 and 8, but for a fourthcore sample; and

FIG. 11 are plots of corrected k_(rwc) values vs. water saturation S_(w)similar to those of FIGS. 5, 7 and 9, but for the fourth core sample.

DETAILED DESCRIPTION OF THE INVENTION

In taking dynamic displacement measurements and in subsequentlycalculating relative water permeability values (k_(rw)) from theacquired data, it can be observed that water relative permeabilityvalues (k_(rw)), calculated using conventional techniques, such as theJones and Roszelle technique, are a strong function of the displacementrate (q_(t)), i.e., fluid flow rate, used in obtaining the dynamicdisplacement data. Variations in the calculated water relativepermeability values can be attributed to the so-called end-effectdiscussed above. Moreover, it can be observed that a critical fluiddisplacement rate (q_(t)) required to eliminate the end-effect over awide angle of saturation values is, for all practical purposes,unattainable.

During investigation of the end-effect on relative water permeabilitycalculations, various dynamic displacement experiments using water-oilsystems were performed on several core samples, designated as P₇, P₂, P₉and 4929B. Different displacement rates q_(t) (e.g., 0.05, 0.10, 0.15,0.20, 0.35, 0.5, 1.0 and 2.0 cc/min) were used for different ones of thecore samples, and relative permeabilities to water k_(rw) werecalculated using the Jones and Roszelle technique. Plots of waterrelative permeability vs. water saturation S_(w) were then produced andare shown in FIG. 4 (for sample P₇), FIG. 6 (for sample P₂), FIG. 8 (forsample P₉) and FIG. 10 (for sample 4929B).

Water relative permeability values were calculated for the fourdifferent core samples at differing oil injection rates q_(t), and waterrelative permeability k_(rw) vs. water saturation (S_(w)) curves werethen produced. The calculations were performed using the conventionalJones and Roszelle technique discussed above. As shown by FIGS. 4, 6, 8and 10, the calculated relative water permeability value k_(rw) changesconsiderably when different displacement rates q_(t) are used. Thedeviations in calculated water relative permeability values k_(rw) areattributed to the end-effect discussed above, which causes an extrapressure drop and distorted effluent measurements at the effluent end ofthe core sample. The width of the end-effect zone increases with adecrease in flow rate, so that the additive pressure drop caused by itcompared with the pressure drop due to the viscous forces will berelatively higher for low displacement rates. Since relativepermeability, as calculated by the Jones and Roszelle technique, isbasically inversely proportional to the pressure drop across the core, alow flow rate results in a calculated relative permeability value whichis too low.

FIG. 1 shows a plot of the ratio of water output flow q_(w) tocalculated water relative permeability k_(rw) vs. the average watersaturation (S_(w)) for the P₇ core sample. An observation of this plotshows that a straight line segment is produced where the oil saturationis relatively high. Experimental results indicated that the range of oilsaturation where the straight line persists increases with displacementrate. Thus, at very low flow rates, the straight line segment isextremely short or non-existent. The deviation of the remaining pointsof the q_(w) /k_(rw) vs. S_(w) plot from the extrapolation of thestraight line increases with a decrease in oil saturation or an increasein water saturation. For the same oil saturation, the deviationdecreases with an increase in displacement rate. Since at relativelyhigh oil saturation the end-effect is insignificant, which coincideswith the observed linear relationship between q_(w) /k_(rw) vs. S_(w),the present invention is based on the theory that the straight linerelationship should be the true relationship between the plot of q_(w)/k_(rw) vs. S_(w), for the total range of saturation values were it notfor the perturbations caused by the end-effect. In other words, thedifference between the experimentally determined ratio q_(w) /k_(rw) andthe corresponding values read from the extrapolated straight line,reflects the perturbations introduced by the extra pressure drop causedby the end-effect. Moreover, if flow is obtained in a stabilized regimewithout the end-effect, the plotted results would terminate at theirreducible minimum water saturation value S_(iw). This is the lowestwater saturation obtainable and is a point where the water flow q_(w)=0.

In the method of the invention, the end-effect is corrected for bycorrecting the experimentally determined ratio q_(w) /k_(rw) to anextrapolated straight line, which is parallel to a straight line segmentin the q_(w) /k_(rw) vs. S_(w) plot for low water saturations (or atangent line to the q_(w) /k_(rw) vs. S_(w) plot) and which is shiftedrelative to the straight line segment or tangent line so as to passthrough the irreducible minimum water saturation point S_(iw).

In the method of the invention, the first step is to conductconventional dynamic displacement tests using an oil and water flood,and from the data obtained, to calculate the relative permeability towater k_(rw) of the core under consideration by the Jones and Roszelletechnique described above and in more detail in the Jones and Roszellearticle referenced above.

Briefly, and by way of background, the method of calculating waterrelative permeability values using the Jones and Roszelle technique isas follows.

Relative oil (k_(ro)) and water (k_(w)) permeability values are definedby the equations:

    k.sub.ro =u.sub.o f.sub.o /λ.sub.2.sup.-1           (1)

and

    k.sub.rw =u.sub.w f.sub.w /λ.sub.2.sup.-1           (2)

where u_(o) is the viscosity of the oil used in the dynamic displacementmeasurements, u_(w) is the viscosity of water used in the dynamicdisplacement measurements, f_(o) is the fraction of oil produced in thecore effluent (0-100%); f_(o) =1-f_(w)), f_(w) is the fraction of waterproduced in the core effluent (0-100%), and λ₂ -1 is an effectiveviscosity. The values f_(o), f_(w) and λ₂ -1 are determined as functionsof saturation.

The effective viscosity λ₂ ⁻¹ can be determined, as per the Jones andRoszelle technique, by taking a tangent to a curve of average effectiveviscosity λ⁻¹ vs. total injected fluid (rate x time) for particularvalues of injected fluid Qi (dimensioned in terms of pore volume), andextending the tangent line to the average effective viscosity λ⁻¹ axis.This extension yields an effective viscosity value λ₂ ⁻¹, which can beused to solve equations (1) and (2) above. Mathematically, the tangentline which yields the effective viscosity λ₂ ⁻¹ is defined by ##EQU1##where Qi represents injected fluid in terms of core pore volume.

The average effective viscosity λ⁻¹ values are in turn defined by thefollowing relationship: ##EQU2## where ΔPb is the pressure drop acrossthe core during a water-only steady state flood of the core sample,q_(b) is the injection rate of the water, u_(b) is the water viscosity,ΔP is the pressure drop across the core during a water flood measurementand q is the water injection rate. The average effective viscosity λ⁻¹obtained from equation (4) is plotted vs. the total amount of injectedfluid Qi, given in terms of core pore volume, and the effectiveviscosity λ₂ ⁻¹ is then obtained, as described above, for any desiredinjected fluid amount Qi.

The other variables in equations (1) and (2) are f_(o) and f_(w). Theseare determined as follows: ##EQU3## with the values λ₂ ⁻¹, f₀ and f_(w),relative permeability values for oil (k_(ro)) and water (k_(rw)) can bedetermined from equations (1) and (2) above.

After water relative permeability values k_(w) are calculated using, forexample, the Jones and Roszelle technique, the ratio q_(w) /k_(rw) isplotted. In plotting q_(w) /k_(rw) an initial water relativepermeability value k_(rwi) for the plotted curve is first determinedusing Darcy's law, which is as follows: ##EQU4## where q_(w) is theeffluent rate, A is the cross-sectional area of the core sample, k isthe specific permeability of the core (measured when only water or oilalone is present), k_(rwi) is a relative permeability to water atresidual water saturation, u_(w) is the viscosity of water, ΔP is thepressure drop across the core at residual water saturation, and L is thelength of the core. All of the values in the above equation are fixed,except for k_(rwi), which can then be calculated. This provides aninitial water relative permeability starting point k_(rwi) for the plotq_(w) /k_(rw). For this point, f_(w) approaches zero and q_(w) /k_(rw)approaches zero.

After plotting the ratio q_(w) /k_(rw) vs. S_(w) using the initialk_(rwi) point calculated from Darcy's law, the plot is evaluated. Itnormally will show a straight line segment in the range of high oilsaturations. This straight line segment is then drawn and extrapolatedto the limit of the experimental data (the plotted q_(w) /k_(rw) vs.S_(w) values). For very low displacement rates, the straight linesegment may not exist. If this occurs, the initial starting point, i.e.,q_(t) /k_(rwi), is used and a tangent line is drawn to the q_(w) /k_(rw)vs. S_(w) ratio plot.

The next step in the method of the invention involves constructing aline in parallel with the straight line segment, or in parallel with atangent to the q_(w) /k_(rw) vs. S_(w) plot if no straight line segmentexists and through the first plotted point, and displaced from thestraight line segment or tangent line so that it passes through theirreducible water saturation value S_(iw), where the flow rate q_(w) =0.This additionally constructed line is illustrated in FIGS. 1, 2 and 3for the same core sample with different flow rates q_(t).

Once the additional straight line is constructed, the corrected relativepermeability values k_(rwc) can then be calculated in accordance withthe expression:

    k.sub.rwc =k.sub.rw (q.sub.w /k.sub.rw)/(q.sub.w /k.sub.rw)SL(8)

where the value (q_(w) /k_(rw)) is the plotted value on the q_(w)/k_(rw) vs. S_(w) curve, calculated using the Jones and Roszelletechnique discussed above, and (q_(w) /k_(rw))SL is the plotted valueread on the additionally constructed straight line, for the same averagewater saturation (S_(w)) value.

The corrected values k_(rws) for water relative permeability are thenused in place of k_(rw) values to produce a curve of water relativepermeability vs. saturation (k_(rw) vs. S_(w)) at the effluent end ofthe core samples.

The method of the invention was used employed with several core samplesto obtain corrected relative water permeability values k_(rw). Dynamicdisplacement measurement data were collected on four core samplesdesignated P₇, P₂ and P₉ and 4929B. The permeabilities of these sampleswere respectively 31, 156, 695 and 25 millidarcies. Several displacementexperiments with variations in rates as high as ten-fold were run oneach core sample. Water relative permeabilities were calculated usingthe conventional Jones and Roszelle technique, and plots of k_(rw) vs.water saturation S_(w) for different water injection rates q_(t) aredepicted for the respective four core samples in FIGS. 4, 6, 8 and 10.The effect of flow rate on the calculated water relative permeabilityvalues k_(rw) is readily apparent from these figures. Thereafter, themethod of the invention was employed, as graphically demonstrated inFIGS. 1 (straight line extrapolation) and 2 and 3 (tangent to curveextrapolation) for the P₇ core samle, to correct the relative waterpermeability values and the corrected results for the four core samplesare respectively shown in the k_(rw) vs. S_(w) plots depicted in FIGS.5, 7, 9 and 11.

It should be readily apparent to those skilled in the art that allprocedures of the above-described method of the invention can beimplemented by a suitably programmed digital computer. In addition,other variations can be made without departing from the spirit and scopeof the invention. Accordingly, the invention is not limited by theforegoing description, but is only limited by the scope of the claimsappended hereto.

I claim:
 1. A method for determining correct relative water permeabilityvalues for a core sample, comprising the steps of:performing oil flooddynamic displacement measurements on said core sample; determiningcharacteristics of said core sample; determining from data collectedduring said measurements, and from said characteristics of saidcharacteristics of said core sample relative water permeability values(k_(rw)) for said core, said determined relative water permeabilityvalues (k_(rw)) being offset from correct relative water permeabilityvalues (k_(rwc)) as a result of an end-effect which introduces anincreased fluid pressure drop at an effluent end of said core sample;and correcting said determined relative water permeability (k_(rw))values toward said correct relative oil permeability values (k_(rwc))thereby minimizing the effects caused by said end effect on saiddetermined relative oil permeability values (k_(rw)), and wherein saidcorrecting step comprises the steps of: initially plotting a ratio q_(w)/k_(rw) vs. average fluid saturation of said core sample, where q_(w) isthe rate of water produced at the effluent end of said core sampleduring said oil flood dynamic displacement measurement; determining aninitial relative water permeability value k_(rwi) at a residual watersaturation of said core sample, said value k_(rwi) being used todetermine the first point of the plot of q_(w) /k_(rw) vs. average fluidsaturation; determining if a straight line segment exists in saidplotted ratio in areas thereof corresponding to high oil saturation and,if so, drawing and extrapolating said straight line segment to the limitof the plotted data, otherwise, constructing a tangent line to theplotted ratio beginning at the point corresponding to k_(rwi) ;producing a straight line in parallel with said straight line segment orsaid tangent line, said produced straight line passing through anirreducible water saturation value S_(iw) ; and determining correctedrelative water permeability values k_(rwc), where ##EQU5## and (q_(w)/k_(rw)) is the initially plotted ratio and (q_(w) /k_(rw))SL is thevalue appearing on the produced straight line for the same average fluidsaturation value.
 2. A method as in claim 1, wherein said average coresaturation is an average water saturation (S_(w)).
 3. A method as inclaim 1, wherein said average core saturation is an average oilsaturation (S_(o)).
 4. A method as in claim 1, wherein said initialrelative water permeability value k_(rwi) is determined in accordancewith Darcy's law.
 5. A method for determining correct relative waterpermeability values for a core sample, comprising the stepsof:performing oil flood dynamic displacement measurements on said coresample; determining characteristics of said core sample; determiningfrom data collected during said measurements, and from saidcharacteristics of said characteristics of said core sample, relativewater permeability values (k_(rw)) for said core, said determinedrelative water permeability values (k_(rw)) being offset from correctrelative water permeability values (k_(rwc)) as a result of anend-effect which introduces an increased fluid pressure drop at aneffluent end of said core sample; and correcting said determinedrelative water permeablity (k_(rw)) values toward said correct relativeoil permeability values (k_(rwc)) thereby minimizing the effects causedby said end effect on said determined relative oil permeabiity values(k_(rw)), and wherein said determined relative water permeability valuesk_(rw) are determined in accordance with the relationship

    k.sub.rw =u.sub.w f.sub.w /λ.sub.2 -1

where u_(w) is the viscosity of the water injected into said core sampleduring said water flood measurement, f_(w) is the fractional flow ofwater from said core sample during said water flood measurements and λ₂-1 is the effective viscosity of the effluent at the outflow end of saidcore sample during said water flood measurements.
 6. A method fordetermining corrected water relative permeability values for a coresample comprising the steps of:injecting a first fluid through said coresample at a predetermined displacement rate for a first predeterminedperiod of time and measuring: (1) a pressure drop of said fluid ΔPbacross said sample after a steady state flow is reached, and (2) a rateof injection of said fluid q_(b) during said predetermined period oftime; determining the total amount of fluid injected Qi in terms of corepore volume; injecting water through said core sample when said coresample is saturated with said fluid and during a second period of timewhile measuring a pressure drop ΔP across said sample; determining afractional water flow f_(w) for said core sample during said waterinjection step; determining average effective viscosity values λ⁻¹ forthe measured values of ΔP, in accordance with the relationship: ##EQU6##where u_(b) is the first fluid viscosity and q is the oil injectionrate; determining effective viscosity values λ₂ ⁻¹, where ##EQU7## andQi is the total amount of injected fluid in terms of core pore volume;determining relative water permeability values k_(rw), where

    k.sub.rw =u.sub.w f.sub.w /λ.sub.2.sup.-1

andu_(w) is the viscosity of the water injected into the core and f_(w)is the fraction of water produced at an effluent end of the core whenoil is injected therein; plotting the ratio q_(w) /k_(rw) vs. averagecore saturation, where q_(w) is the amount of water contained in theeffluent discharging from said core sample during said oil injection;determining an initial relative water permeability value k_(rwi) at theresidual water saturation of the core sample, said value k_(rwi) beingused to determine the first point of the q_(w) /k_(rw) vs. average coresaturation plot; determining if a straight line segment exists in saidplotted ratio in areas of the plot corresponding to high oil saturationand, if so, extrapolating said straight line segment to the limit of theplotted data, otherwise, constructing a tangent line to the plottedratio beginning at the point corresponding to k_(rwi) ; producing astraight line in parallel with said straight line segment or saidtangent line, said produced straight line passing through an irreduciblewater saturation value S_(iw) ; and determining corrected relativepermeability values k_(rwc), where ##EQU8## and (q_(w) /k_(rw)) is theinitially plotted ratio and (q_(w) /k_(rw))SL is the value appearing onthe produced straight line for the same core saturation value.
 7. Amethod as in claim 6, wherein said average core saturation comprises atleast one of an average water (S_(w)) saturation and an average oil(S_(o)) saturation.